I have now checked all the users in the system to find those that have incorrect ranks and found the following:

1. Users who had wrong levels did not have double levels by default as it seems. The double levels came since they reported to me that they have the wrong level and I then promoted them, which wrongly left the old level around and gave them the new level on top of that – which it should not. I need to find out why that is. This issue could be fixed by migrating to another permissions plugin.
2. There were a number of users (6 or so) which had the wrong level and I manually now set the right level and made sure they don’t have several. The big question now is if the problem reoccurs or not.

If we are lucky, then this was a one-off and simply was not properly fixed (see point 1). I have a hunch however that this will happen again. Please let me know if you see yourself not having the correct userlevel so I can investigate further on this.